1. Field of Invention
This invention relates to molds for the manufacture of hollow blown glass and lead crystal glassware and a unique process for making such molds. The invention has particular application for making relatively inexpensive molds, capable of repeated cycles of use, for the manufacture of glass shapes having non-round cross-sections and having a high quality of glass clarity, color and smooth surfaces, all indicative of quality glass production.
2. Description of the Prior Art
In the mass production of hollow glassware items, such as drinking glasses, bowls, vases and the like, the methods of manufacture are primarily two categories: blown and pressed production, either manual, semi, or fully automatic in character.
To mass produce glass shapes having both round and non-round sections, and which will be of a consistent shape, with high quality of clarity, color and surface quality, molds are required for both the blowing and the pressing of these pieces.
In the mass production blowing of round, or circular items, the current method for producing molds consists of carving out a cavity from a block of fresh (i.e. wet) wood, to the desired profile.
It is well known in the art of producing round glassware items to employ such negative molds fabricated from fresh fruitwoods, such as pear wood, (sometimes called "soft tools"), which are maintained sufficiently wet within the mold cavity to permit molten glass having temperatures of 1200.degree. C. to 1900.degree. C. to be placed within the mold and blown to its shape using a constant rotating motion. The molten glass causes the residual moisture in the mold to boil off as steam, creating a vapor layer of steam inside the mold which, with the usual turning and blowing technique well-known in the prior art, will produce a smooth surface and quality glass piece.
Such molds can be either be of one-piece construction, or of multiple molds which are then hinged and supplied with handles for the opening and closing of the mold in order to afford the ingress and egress of the parison. These wood molds are inexpensive to produce, owing particularly to the relatively low cost of the wood, the simple machinery required for fabrication, and the availability of skilled labor for this purpose.
As well known in the art, these wood molds are maintained in the wet state by being constantly immersed in a bath of water when not in use. This serves a number of purposes. First, it reduces the burning of the mold in use, and subsequent cracking, as well as contributing to the maintenance of the roundness of the mold which can last from 200 to 600 production cycles. In addition, the bath provides the residual moisture in the mold, so that when the hot parison is inserted for blowing, steam evaporates along the inside walls of the mold and around its circumference, as has been described. This layer of steam in concert with the rotational action on the parison itself are the two major contributing elements resulting in superior surface quality, owing to the fact that both these result in the parisons never making intimate contact with the inside of the mold. It is this thin layer of steam which imparts the smooth surface to the piece.
It is commonly known in the art that when a glass piece is to be produced in a quantity in excess of about 5000, it is generally more cost effective to produce a steel mold, which offers a useful life in the tens-of-thousands-piece range. For round pieces, this steel mold presents no particular problems of fabrication of the shape as it can be simply produced using common machine tools.
It is well-known that a smooth mold surface is preferable, although the quality of the mold surface need not be the most polished for pieces which will be heavily decorated with cuts, either polished or not, since the cuts will in effect hide most of the surface defects. If, on the other hand, the blown piece will remain undecorated, then the quality of the finished piece is measured by the absence of defects particularly in regard to color, clarity, consistency of surface, and content of foreign objects such as stones, pits and the like. For such undecorated pieces, a mold of the highest quality is required, one which has a highly polished and defect-free surface. Heretofore, these prior art steel molds have been manufactured from a steel alloy offering the least corrosive or oxidating effect on the glass. Such steel alloys are of limited availability.
It is known in the art that the molds must be cooled between cycles. Use of a hot mold tends to cause the glass to stick within the mold cavity, resulting in the destruction of the surface. Unlike the wood molds and the molds of the present invention, the steel molds cannot be cooled with water. The steel molds are cooled with oil, kerosene, or air which is applied directly to the mold surface. Use of air alone slows production and the liquid coolants present problems in the finished blown pieces in that it is quite difficult to rid the glass of these oils. For example, an air-cooled glass molding mechanism for making round cross-section pieces is disclosed in U.S. Pat. No. 3,374,079.
As is apparent from the foregoing description of the prior art, molds for a rotating method of production can be successfully used only when the cross-section of the mold, in a plane generally perpendicular to the axis of rotation, is uniformly round so as to permit rotation of the parison to maintain a layer of steam at all points between the parison and the mold during the rotation and blowing. This rotation method for round cross-section pieces is described for example, in U.S. Pat. Nos. 3,271,123 and 3,374,079 which do not disclose any particular mold material.
A mold for forming glass pieces by the process of sagging a flat sheet of glass is disclosed in U.S. Pat. No. 4,059,428. This patent does not disclose a pourable or room temperature molding composition, as disclosed in the present invention. The process in U.S. Pat. No. 4,059,428 is also not adapted to form molds for non-round shapes, but requires grinding and high temperature heating of a green-glass blank.
When producing blown glass pieces which are non-round in cross-section, such as a square vase, the known methods and tools change dramatically. Due to the non-round shape, the rotating action cannot be used and the molten glass must necessarily come into intimate contact with the mold surface, and maintain such relationship for an extended period of time while the glass cools sufficiently to hold its shape. Instead of a rotary motion applied to the blowpipe, the blower must rely on his skill in the art of blowing, and requires a mold with an inner surface as flawless and perfect as possible, and capable of withstanding contact with the molten glass for two to three times as long as in forming round pieces. The blower must maintain contact between the glass and inner surface of the mold while the glass cools enough to hold the shape, at the same time maintaining uniform wall thickness. This is a longer and more difficult process than for forming round pieces by the rotation method.
Since the fresh wood molds cannot withstand such extended exposure to heat for any economical number of production cycles, it is well known in the prior art to manufacture molds for non-round articles from stainless steel alloys having a low rate of oxidation and distortion when exposed to the glass blowing temperatures. These molds can be highly polished or plated to obtain the proper surface quality and may have service lives of as many as 50,000 production cycles. Fashioning molds from stainless steel alloys is much more expensive and time consuming than fashioning wood molds and poses problems of capital investment and lead time of at least 8-16 weeks to fabricate a steel mold for each new non-round shape. In addition, the stainless steel molds require periodic maintenance, ranging from simple repolishing, to complete replating.
In contrast to the steel molds for round shapes, which can be easily machined from stock on a lathe, a steel mold for a non-round shape must be machined from stock on highly sophisticated milling equipment or through the use of electrode burning. Along with polishing or plating, such molds require substantial time for fabrication, on the order of three to five months and at much greater expense than wood or round steel molds.
Commercial and marketing considerations are a major factor in the decision to undertake such fabrication expenses for new non-round glass shapes. Before making an investment the size of which is required for non-round steel molds, the manufacturer usually prefers to make some sample production using inexpensive molds having limited life.
In the case of round items, there is no problem, since if the samples are successful, the wood molds made with little investment can also be used for full scale production with perfectly satisfactory results, even though it may be necessary to make a number of them to continue production until steel molds are available.
In the case of new non-round shapes, however, although such soft tools can be used to produce five to ten samples, this limited supply may be insufficient for test marketing. Moreover, if the samples are successfully marketed, the long lead time to produce stainless steel molds needed for full scale production prevents the manufacturer from immediately capitalizing on the new shape he introduced and allows time for the shape to be copied and marketed by other manufacturers.
These known problems are exacerbated by the commercial reality that manufacturers will usually desire to test market a new line or collection of shapes and sizes of from fifteen to forty different pieces, many of which may be non-round. Accordingly, for such pieces the commitment of time and money for stainless steel molds must often be made at the earliest stage of marketing just to produce samples, and long before any commercial determination of the saleability of the shape. The business risks in such investment are apparent.
It has been known in some applications, primarily for foundry casting of molten metals, to attempt to make non-steel or pourable refractory molds. For example, certain resin-bonded sand molds have long been known in the foundry industry for producing metal castings, as disclosed, for example, in U.S. Pat. Nos. 2,869,194, 3,005,244, 4,121,646 and 4,336,179. These molds have generally been non-permanent molds in which the resin is burned out by the heat of the molten metal and the mold must be physically broken away from the cast metal piece, thus destroying the mold after each casting and imposing the time and expense of repeated mold making. Such foundry molds also do not provide an as-molded surface of sufficient quality for glass production.
U.S. Pat. No. 3,802,891 discloses a semi-permanent foundry mold composed of wollastonite, a mineral, and a silicious binder which is heated in the range of 2,200.degree. F. to sinter the mold material to final strength. This appears to be limited to a foundry application rather than for producing the high quality surfaces needed for forming glass pieces or for forming a permanent mold capable of producing over 1200 glass pieces as disclosed in the present invention.
Other attempts to produce quality glass, as in the art of non-hollow glass optical elements, include use of fine grained silicon carbide or silicon nitride on a substrate or as the mold composition, as disclosed, for example, in U.S. Pat. No. 4,139,677. This method also requires pre-grinding and polishing of the mold surfaces, and in production requires optical quality glass blanks, induction heating of the molds in a controlled, non-oxidizing atmosphere for press molding with close temperature monitoring throughout the production cycle.
A method of mold making used primarily for soft metal foundry sample molds for temperature applications up to 3000.degree. F. (1600.degree. C.) involves forming the mold from a thermoplastic-thermosetting resin compound and amorphous fused silica using proportions and curing far different from that disclosed in the present invention. Kits containing these pre-measured components for making such foundry molds are sold commercially by A.C.E. Inc. of Plainview, N.Y. The material has also been reported by A.C.E. Inc. for use in shaping glass sheets or cast molten metal apparently for limited production cycles. However, according to the material description provided by A.C.E. Inc., the resulting mold must be "machined" "painted or refinished" to produce smooth mold surfaces.
As disclosed by A.C.E., Inc. the mold materials in the kit must be mixed in the weight ratio 5 lbs. of hydraulic compound powder to about 1.42 lbs. of water to 6.5 lbs. of amorphous fused silica or in a larger quantity but by the non-linear increases in proportions to the ratio 22.8 lbs. of compound to 7.4375 lbs. of water to 34.2 lbs. of amorphous fused silica. Without a kit, A.C.E. Inc. recommends a ratio of 4 lbs. of compound to 1/3lbs. of water to 6 lbs. of amorphous fused silica. The first mix, compound and water, is to be machine roll mixed at 60 R.P.M. for 5 minutes and this composite is then roll mixed with the amorphous fused silica for 10 minutes at 60 R.P.M., followed by pouring the mold and using only a room temperature cure.
Significantly, A.C.E. Inc. teaches the criticality of the disclosed ratios and recommends against roll mixing the smaller quantity kit, noting "[d]ue to accuracy required, Factory does not recommend this procedure." In addition, it appears that the A.C.E. Inc. molds are not designed for applications at temperatures in excess of 1600.degree. C., whereas the molds of the present invention are routinely used at temperatures as high as 1940.degree. C.
Accordingly, none of the known methods for making non-steel molds has provided molds with the quality, economy and permanance of the present invention for economically producing molds for the manufacture of quality non-round glass pieces in quantity from a single mold.